Generally it is known from molecular biology that, for the production of specific compounds, microorganisms are transformed with a so-called "artificial" plasmid, in that the genes, which are coded for this specific compound, are introduced. A special problem of these plasmids is their stability, i.e., their property not to be transmitted in a controlled way to the daughter cells during the cell division of the microorganisms. The result is that more and more daughter cells occur during the fermentation process that contain no plasmid or fewer plasmids.
On a laboratory scale this plasmid loss can be countered by supplying the antibiotic to the culture medium whose corresponding gene resistant to antibiotics contains the plasmid. However, the addition of the appropriate antibiotic in fermentations on a large scale has proven to be disadvantageous. Thus, for example, some antibiotics, such as, tetracycline, show unfavorable effects on the ability of microorganisms containing the plasmid to grow, divide and reproduce [Bioscience Reports, 5, (1985), pp. 29-37; Gene, 39, (1985), pp. 173-180]. Another drawback of antibiotic stabilization lies in that the addition of an antibiotic, especially in fermentation on a large scale, is too expensive. Further, the addition of an antibiotic in the production of pharmaceutical agents as well as in the production of food and feed additives is undesirable or unlawful.
Another method of how to counter this plasmid loss is described by H. Sakoda and T. Imanaka in J. Ferment. and Bioeng., Vol. 69, (1990), pp. 75-78. This method comprises a stable "recombinant host" plasmid system, in which first the tryptophan operon in the chromosome of the host is deleted and thus the host cell is inactive for the tryptophan transport. Then the host cell is transformed with a recombinant plasmid which carries this tryptophan operon. The selection of the host cells then takes place with this recombinant plasmid by the tryptophan transport. The drawbacks of this method lie in that, despite selection with tryptophan, in the actual fermentation process plasmid-free cells can also grow because of diffusion and thus daughter cells which contain no plasmid increasingly occur.